Achromatic Passive Q-Plates Using Multilayer Liquid Crystal Photoalignment

Q-plates convert spin angular momentum to orbital angular momentum using a spatially varying half-wave retardation, but conventional devices are strongly wavelength dependent. This work demonstrates a passive approach to reduce chromatic sensitivity using three photoaligned liquid-crystal layers. The tandem design requires a fixed fast-axis rotation of the middle layer relative to the outer layers, implemented directly in the photoalignment pattern. Devices were fabricated using SD1 photoalignment with SLM-based polarization patterning and E7 liquid crystal cells with 1 micrometer thickness per layer. Polarized optical microscopy confirms the intended interlayer rotation and the designed plus-one q-plate order. When operated at 443 nm (away from the single-layer design wavelength), the three-layer stack yields a higher-contrast donut profile with a deeper central minimum compared with a single layer.